1. Field of the Invention
This invention relates to a method of heat treating raw material components for the production of cement clinker. More particularly, this invention relates to a method of separately heat treating components of the raw material.
2. Description of the Prior Art
Raw material for the production of cement, such as Portland cement, is comprised of a pulverulent mixture of a calcareous component (such as limestone) as a source of calcium oxide, and an argillaceous component (such as clay or a mixture of clay and sand) as a source of oxides of aluminum, silicon, iron and other minerals. The raw materials are processed into cement clinker by drying, dehydrating, calcining and sintering the materials. Calcination is a reaction during which carbon dioxide within the calcareous component (in the form of calcium carbonate) is liberated leaving a free lime which sinters with the dehydrated argillaceous material to form clinker. The calcination reaction is endothermic while the sintering reaction is exothermic.
Historically, the processes for making a cement clinker have been performed almost exclusively in rotary kilns. The raw material is fed to an inlet end of the kiln and progresses to a discharge end. A burner positioned at the discharge end supplies the necessary heat to maintain the process reactions as the material moves through the kiln. Generally, the burner was required to generate at least 800 kcal per kg of clinker produced for a dry process.
Under historical processes, in order to obtain higher output capacities, the kilns were made progressively larger. This practive resulted in various inefficiencies such as increased radiation losses from the kiln shell and short life of the kiln's refractory lining. Additionally, the practice of carrying out the total cement manufacturing process in the kiln had the inherent inefficiency that heat generated by the burner was being generated in a location away from the greatest heat need. The burner was generating heat in the region of the kiln where the sintering reaction (an exothermic reaction) was occurring while the generated heat was needed in the region of the kiln where the calcination reaction (the endothermic) reaction was occurring.
Cognizant of the above, as well as other problems associated with the complete manufacture of cement in a kiln, cement manufacturers developed processes by which a substantial portion of the heat treatment processes, including the calcination reaction, occurred prior to the material being fed into the kiln. Consequently, the kiln needed to generate only the amount of heat needed to maintain the sintering reaction.
Such heat treatment processes comprised a suspended heat exchanger in material flow communication with the inlet of the kiln. The heat exchanger typically comprised a plurality of series connected cyclones for preheating and dehydrating a pulverulent mixture of the raw material and a calcining furnace, or the like, for receiving the dehydrated material and at least partially calcining the mixture prior to admitting the mixture to the kiln. Examples of such processes may be found in U.S. Pat. No. 3,914,098 to Kano et al, dated Oct. 21, 1975.
While such suspension-type preheating processes advanced the art of cement manufacturing, they are limited to the extent they can relieve the kiln of its heating burden. A chief cause of the limitation is, if the raw materials are heated to too great an extent in the preheater, the materials begin to undergo a phase change where the materials become adhesive and adhere to the preheater equipment. The adherence interferes with both material and gas flow through the equipment.
The desire to relieve the kiln of its heating burden is a continuous one. Additionally, the desire to heat as high as possible in the suspension preheaters is particularly desirable in processes where the clinker is to be sintered in a reactor other than a rotary kiln (such as in an electric arc furnace) where the costs of generating the heat in the reactor are particularly high. Such a process may be as described in U.S. Pat. No. 3,203,681 to Rosa et al, dated Aug. 31, 1965.
We have determined separately heat treating the raw material components of the cement clinker can be accomplished in a suspension-type preheater with at least one of the components heated to a temperature in excess of the temperature needed to initiate sintering and without the troublesome phase change noted above. Accordingly, the raw material component supplies at least a portion of the heat needed to maintain sintering when mixed with the other components. The sintering reactor is required only to augment the heat of the raw material component as needed.
Previously known processes involving at least a partial heat treatment of separate raw materials may be found in the following U.S. Pat. Nos.: 3,973,980 to Rohrbach et al, dated Aug. 10, 1976; 4,256,502 to Lovichi et al, dated Mar. 17, 1981 and 3,139,463 to Wuhrer, dated June 30, 1964.
Rohrbach teaches preheating a clay component in a preheater with waste gas from a kiln. A limestone component is fed to a separate preheater to which a flow of hot gas is supplied to preheat the limestone. The preheated clay and limestone are fed past burners which calcine the limestone with the mixed calcined limestone and clay fed to a kiln for sintering. The process as taught by Rohrbach reduces the heat requirement of the kiln to 100 kcal per kg of clinker.
Wuhrer teaches drying a clay component with waste gas from a kiln. The dry clay is mixed with lime and the mixture is preheated with hot air from a cement clinker cooler. The preheated mixture is fed to a kiln for sintering. Lovichi discloses a process where shale is precalcined in a separate preheater and mixed with a preheated mixture of clay and limestone from a preheater with the mixture of calcined shale, clay and limestone fed to a kiln for sintering.
In each of the above described processes, the extent to which the sintering reactor (the kilns in the disclosed processes) can be relieved of a heating burden is limited. The amount of heat which can be added to the material prior to the admission to the kiln is limited by the temperatures at which the mixture enters a phase where the material is adhesive. Practically, this temperature is the temperature at which the calcareous component calcines.